I am not super familiar with Oberlin, but from what I have seen, many LACS defer a lot of students into the RD round, and some do get accepted. I suspect that the schools feel they have to take certain ones (athletes, legacies, super stars, other hooked) to build out the frame if the class, but that they then want to look at the whole pool before extending acceptances in the spring.
I don’t know about the conservatory angle. It sounds like your D is doing all the right things. Make sure she let’s them know of any prizes, accomplishments, etc. It also means that if she finds something else she likes better along the way, she is no longer locked in.
Sorry for your early disappointment. Hang in there!